God performs a miracle in each Mass. In fact, there has never been a miracle as great as this anywhere on earth for 2000 years. And it happens in every Catholic church every day!

“‘It is not man that causes the things offered to become the Body and Blood of Christ, but He who was crucified for us, Christ himself. The priest, in the role of Christ, pronounces these words, but their power and grace are God’s. This is my body, he says. This word transforms the things offered” (St. John Chrysostom; CCC 1375). “This change is not like natural changes, but is entirely supernatural, and effected by God’s power alone” (St. Thomas Aquinas, Summa Theologiae III, 75,4).

Such a miracle is beyond the power of man, but not beyond the power of God. “‘ . . . Could not Christ’s word, which can make from nothing what did not exist [Gn 1], change existing things into what they were not before? . . .” (St. Ambrose; CCC 1375).

Reason says it is possible. But faith says it is actual.
Why do Catholics believe this astonishing fact – that what seems to all human perception to be ordinary bread and wine is in fact the body and blood of God incarnate? Because Christ said so! “‘Because Christ our Redeemer said that it was truly His body that He was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change in the whole substance [being, essence] of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of His blood. This change the holy Catholic Church has fittingly and properly called transubstantiation” (Council of Trent in the sixteenth century; CCC 1576).

“The Eucharistic presence of Christ begins at the moment of the consecration and endures as long as the Eucharistic species subsist . . . ” (CCC 1377). Since they remain for about 15 minutes in the human body after being swallowed, we should spend this time in prayer, thanksgiving, and adoration, and not quickly turn to worldly occupations.